There are two important rationales for the sensitizing of cells to ionizing radiation. Either approach could eventually prove efficacious in the treatment of cancer by radiation. One rationale requires incorporation of a sensitizing molecule, such as bromouracil, within the nucleic acid structure of the proliferating cells. The molecular mechanisms of this mode of sensitization will be investigated using ESR- ENDOR spectroscopy. Primary and secondary steps in the radiation damage process will be studied in appropriate halogenated pyrimidine derivatives, irradiated at liquid helium temperature (4.2 degrees K). The effect of low temperature is to stabilize the primary phase of the radiation damage process. Similar techniques will be used to investigate those sensitizers which supposedly function according to the second rationale, namely, by mimicing the effect of oxygen. Chief among the compounds to be investigated in this category are nitro derivatives, such as p-nitroacetophenone, which have demonstrated sensitizing ability in irradiated cell cultures.